Cancer is recognized by uncontrolled growth of cells, which form a tumor and ultimately may invade other tissues (metastasis). Cancer affects people of all ages, and is a major cause of human death whereby in particular lung cancer, stomach cancer, colorectal cancer and liver cancer are the deadliest ones. The most commonly occurring cancer in men is prostate cancer and in women it is breast cancer.
Each year 11 million people worldwide are diagnosed with cancer. Almost 7 million die from the disease. Treatment options of many cancer types and the success rate of intervention largely depend on the stage of the disease at the time of diagnosis. In many cases, early detection is of utmost importance as it greatly increases the chances for successful treatment. The implementation of screening programs and tools for early detection and better diagnosis is of great importance. However, this also increases the detection rate of latent and clinically irrelevant tumors. This poses a great risk of overtreatment and is therefore a tremendous clinical dilemma since no effective tests for disease progression are available. On the other hand, a substantial number of patients will develop clinical metastatic disease or already presents with occult metastases at the time of staging. Moreover, tumors often respond differently to a given type of therapy and often acquire resistance to therapy. These situations emphasize the need for reliable criteria and tools for treatment decision and follow-up in order to implement a more personalized and adequate treatment.
Currently, cancer diagnosis, cancer subtyping and treatment decision of many cancers is largely based on the histopathological TNM (tumor, node, metastasis) system, the histological differentiation of the primary tumor, and immunohistochemistry. As these systems provide only an ad hoc picture of the tumor based on few parameters, multiparameter molecular-based insights in the tumor characteristics will undoubtedly help to more accurately predict progression and therapy response and will aid in the selection of more appropriate primary and/or adjuvant treatment options. Recent advances in molecular analytical methods hold significant promise in this respect and have the power to revolutionize the ways cancer is diagnosed and treated.
Up to now, most efforts towards multiparameter molecular characterization and staging of cancer are carried out at the level of DNA, RNA or proteins (screening for genomic mutations, high throughput genome or exosome sequencing, analysis of epigenetics, transcriptome analysis, or protein profiling). For example, gene expression profiling can predict clinical outcomes of prostate cancer (G. V. et al., J. CHn. Invest. 113: 913-923 (2004)) as well as breast cancer (Van't Veer et al. Nature 415: 530-536 (2002)).
Nevertheless, changes more downstream are perhaps of more significance as they represent more distal endpoints of cellular regulation, integrating diverse (epi)genetic, regulatory and environmental cues. Of particular interest in this respect are changes in membrane lipid composition.
Functioning as barriers that separate and compartmentalize the cell's content, membranes function as unique interfaces at which numerous cellular processes (including signaling, nutrient transport, cell division, respiration, cell death mechanisms, etc) are concentrated and regulated. An ever increasing body of evidence indicates that membrane lipids, and particularly changes in phospholipid species play a central role in this regulation (Marguet D, Lenne P F, Rigneault H, He H T (2006) Dynamics in the plasma membrane: how to combine fluidity and order. EMBO J 25: 3446-3457).
Phospholipids are a complex class of cellular lipids that are composed of a headgroup (choline, ethanolamine, serine, inositol, etc) and 1 to 4 fatty acyl chains that can differ both in length and in the number of unsaturations (double bonds), leading to hundreds of different species. The building blocks for these lipids can be taken up from the circulation, however, some can also be synthesized de novo. Most cells express elaborate pathways that dynamically modify lipid structures. This can change their chemical properties dramatically and locally modulates the biochemical and biophysical properties of membranes.
There is mounting evidence that in tumors, phospholipid metabolism is dramatically different from normal tissue. Whereas most normal tissues acquire the bulk of the required lipids from the circulation, tumor cells frequently synthesize the majority of their lipids de novo (Brusselmans, K., and Swinnen, J. V. (2009) The lipogenic switch in Cancer. In Mitochondria and Cancer, K K Singh and L. C. Costello, Eds, Springer, New York, USA pp. 39-59). This is illustrated by a dramatic overexpression of lipogenic enzymes such as fatty acid synthase in tumors, particularly in those with a poor prognosis. Activation of this lipogenic pathway involves changes at all levels of enzyme regulation (genetic changes, enhanced transcription and translation, protein stabilization and phosphorylation, allosteric regulation and substrate flux) and occurs downstream of various common oncogenic events (loss of PTEN, activation of Akt, loss of BRCA1, steroid hormone action, tumor-associated hypoxia, etc.) (Swinnen, J. V., Brusselmans, K., and Verhoeven, G. (2006). Increased lipogenesis in cancer cells: new players, novel targets. Curr Opin Clin Nutr Metab Care 9, 358-365).
Conversely, other tumor types or tumor subtypes seem to activate the uptake mechanisms of fatty acids for instance by expressing lipoprotein lipase (Kuemmerle N B, Rysman E, Lombardo P S, Flanagan A J, Lipe B C, Wells W A, Pettus J R, Froehlich H M, Memoli V A, Morganelli P M, Swinnen J V, Timmerman L A, Chaychi L, Fricano C J, Eisenberg B L, Coleman W B, Kinlaw W B. Lipoprotein lipase links dietary fat to solid tumor cell proliferation. Mol Cancer Ther. 2011 March; 10(3):427-36). Besides the provision of fatty acids for cell proliferation, the impact of the increased fatty acid synthesis versus uptake has remained unknown. In addition to changes in de novo fatty acid synthesis and uptake, there is evidence also for other changes in lipid metabolism, including changes in the expression of phospholipases, COX2, and ELOVL7, an enzyme involved in the elongation of saturated long chain fatty acids (Novel lipogenic enzyme ELOVL7 is involved in prostate cancer growth through saturated long-chain fatty acid metabolism. Tamura K, Makino A, Hullin-Matsuda F, Kobayashi T, Furihata M, Chung S, Ashida S, Miki T, Fujioka T, Shuin T, Nakamura Y, Nakagawa H. Cancer Res. 2009 Oct. 15; 69(20):8133-40.) Besides its role in the production of cholesteryl esters required for steroid hormone synthesis in prostate cancer, the extent of changes in elongation in cancer development by this and by other members of the ELOVL family, its impact on membrane phospholipid composition and its role in cancer development and progression have remained unknown.
Correlations between dietary fatty acids and the risk of developing cancer have been described previously. For example, increased intake of particular n-6 polyunsaturated fatty acids (Godley, P., Campbell, M., Gallagher, P., et al. (1996). Biomarkers of Essential Fatty Acid Consumption and Risk of Prostatic Carcinoma. Cancer Epidemiology, Biomarkers & Prevention 5, 889-895), saturated fatty acids (palmitic acid; Harvei et al., 1997)(myristic acid; Mannisto et al., 2003), and monosaturated fatty acids (palmitoleic acid; Harvei, S., Bjerve, K., Tretli, S., et al. (1997). Prediagnostic level of fatty acids in serum phospholipds: omega-3 and omega-6 fatty acids and the risk of prostate cancer. In. J. Cancer 71, 545-551) have shown association with an increased risk of developing cancer. Furthermore, dietary fatty acids have also been shown to correlate with the aggressiveness of tumors. For example Crowe et al. showed that palmitic acid (saturated fatty acid) was positively correlated with low-grade (less aggressive) prostate cancer, whereas myristic acid (saturated fatty acid) as well as linolenic acid and eicosapentaenoic acid (both n-3 polyunsaturated fatty acids) were positively correlated with high-grade (aggressive) prostate cancer (Crowe, F., Allen, N., Appleby, P., et al. (2008) Fatty acid composition of plasma phospholipids and risk of prostate cancer in a case-control analysis nested within the European Prospective Investigation into Cancer and Nutrition. Am. J. Clin. Nutrition 88: 1353-1363).
Further to these known correlations of dietary fatty acids and cancer prediction/prognosis, we have now found that cancer development is often accompanied by changes in acyl chain length of phospholipids, in particular with a relative increase in the fraction of longer phospholipid species when compared to the fraction of shorter phospholipid species within at least one head group class.
We demonstrate that these changes affect the proliferation and invasiveness of cancer cells. We also show that acyl chain length is modulated by cancer treatment for instance by tyrosine kinase inhibitors, such as imatinib. These findings indicate that membrane phospholipid profiling can be used for the development of diagnostic, prognostic as well as predictive and follow-up tests for assessing the evolution and therapy response of a tumor in a subject. It will also aid in the selection and follow-up of patients benefitting from treatments targeted to the lipid metabolic enzymes themselves, including inhibitors of lipogenesis, elongation and other lipid metabolic processes.
Implementation of the use of such phospholipid profiles as diagnosic/predictive/prognostic biomarkers will lead to a more personalized medicine in which diagnosis and treatment are more interdependent and based on molecular evidence of how a tumor will evolve and respond to a particular treatment. These advances will allow the physician to tailor the treatment to the patient's individual needs. It will also avoid excess of morbidity and side effects due to overtreatment (i.e. sparing the patient from an invasive surgical procedure in case of nonaggressive or too advanced disease), and will optimize the use of available resources.